The US president Barack Obama and his Secret Service agents during a trip. Photo: Reuters

A sex scandal involving Secret Service agents on US President Barack Obama's trip to Colombia appeared to grow Sunday after visiting American soldiers were also linked to the use of prostitutes.

Eleven members of the service, known for their robust handling of security for senior members of the US government, were suspended and sent home following allegations of misconduct in the resort city of Cartagena.

Five US military personnel are also being investigated for behavior said to have taken place at the same hotel where the Secret Service staff were staying, and they have been taken off duty and confined to barracks.

Obama arrived late Friday in the Caribbean port of Cartagena, under security lockdown for the Summit of the Americas, but the allegations against the Secret Service and military completely overshadowed the gathering of regional leaders.

The misconduct claims were made on Thursday against the Secret Service personnel, who included both special agents and uniformed division officers, though none of them was assigned to Obama's personal security detail.

Secret Service Assistant Director Paul Morrissey said "the nature of the allegations, coupled with a zero tolerance policy on personal misconduct," led to the "decisive action to relieve these individuals of their assignment."

Having been sent back to the United States, the personnel involved were taken to the service's Washington headquarters for interviews on Saturday as part of the agency's internal affairs division's investigation.

"As a result, all 11 employees have been placed on administrative leave. This is standard procedure and allows us the opportunity to conduct a full, thorough and fair investigation into the allegations," Morrissey said.

The Washington Post, quoting Jon Adler, president of the Federal Law Enforcement Officers Association, reported that at least one agent had been involved with prostitutes in Cartagena.

Ronald Kessler, a former Post reporter and author of a book on the Secret Service, told CNN the scandal erupted when one of the agents "did not pay one of the prostitutes, and she complained to the police."

Meanwhile, Republican Representative Peter King, who chairs the House Homeland Security Committee, told The New York Times that the 11 agents were suspected of bringing women back to their rooms.

While prostitution is legal in designated areas in Colombia, such behavior would violate agency rules of conduct, in part because it could expose the agents to blackmail, facilitate espionage and help an enemy get inside a security perimeter, said King, according to The Times.

White House spokesman Jay Carney told reporters that Obama had full confidence in the Secret Service.

Shortly after, a statement released by the US military's Southern Command, tasked with planning and operations in Central and Southern America, said five of its staff had been linked to the same misconduct claims in Cartagena.

Its commander, General Douglas Fraser, was quoted to be "disappointed by the entire incident and that this behavior is not in keeping with the professional standards expected of members of the United States military."

He went on to say that "after a thorough investigation, punishment, if appropriate, will take place," in accordance with military justice.

The revelations came as two small bombs exploded in the capital Bogota, near the US embassy, and another two in Cartagena, though no one was hurt and there was no damage.

The Secret Service, which employs some 3,200 agents and 1,300 uniformed police, has been in the spotlight for a number of notable incidents since Obama took office three years ago.

In late 2009 investigations were launched after an uninvited couple -- both aspiring reality TV stars -- gatecrashed Obama's first state dinner at the White House, gaining access to the party and even getting photographed with the president.

The couple from Virginia, Tareq and Michaele Salahi, made headlines after attending the early part of the dinner honoring visiting Indian Prime Minister Manmohan Singh despite lacking an invitation or Secret Service clearance.

In November last year an agent was charged with second-degree murder after an incident in Hawaii in November ahead of an APEC summit, The Washington Post reported, while another was charged with drunk driving last August while helping to arrange security for an Obama bus-trip in Iowa.